#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long int ll;
typedef long double ld;
#define FOR(i,l,r) for(ll i=l;i<r;i++)
#define REP(i,n) FOR(i,0,n)
#define RFOR(i,l,r) for(ll i=r-1;i>=l;i--)
#define RREP(i,n) RFOR(i,0,n)
#define ALL(x) x.begin(),x.end()
#define P pair<ll,ll>
#define F first
#define S second
#define BS(A,x) binary_search(ALL(A),x)
#define LB(A,x) (ll)(lower_bound(ALL(A),x)-A.begin())
#define UB(A,x) (ll)(upper_bound(ALL(A),x)-A.begin())
#define COU(A,x) UB(A,x)-LB(A,x)
template<typename T>using min_priority_queue=priority_queue<T,vector<T>,greater<T>>;
using mint=modint1000000007;
signed main(){
  ll N,s=0;cin>>N;vector<ll>L(N),S(N);REP(i,N)cin>>L[i]>>S[i];
  dsu D(N);REP(i,N)D.merge(i,S[i]-1);REP(i,N)s+=L[i];
  vector<vector<int>>Q=D.groups();vector<bool>X(N,0);
  REP(i,Q.size()){
    ll v=Q[i][0],m=1e18;while(!X[v]){X[v]=1;v=S[v]-1;}
    vector<ll>roop;while(X[v]){roop.push_back(v);X[v]=0;v=S[v]-1;}
    REP(j,roop.size())m=min(m,L[roop[j]]);s+=m;
    //cout<<Q[i][0]<<" "<<m<<endl;
  }
  if(s%2)cout<<s/2<<".5"<<endl;
  else cout<<s/2<<".0"<<endl;
  return 0;
}